When gamma ray spectra are obtained from scintillator detectors in logging tools and other devices, it is useful for the spectra to include a standard gamma ray peak or line at a known energy for use in determining the energy calibration and resolution performance of the observed spectra.
One way in which such a reference line has been provided in the past has been to utilize a naturally occurring or neutron-induced gamma ray energy line from the materials in the surrounding environment, i.e., external to the detector device. In U.S. Pat. No. 3,829,686, for example, the gain of a borehole gamma ray energy measuring system is controlled by use of the 2.23 MeV hydrogen line peak in the thermal neutron capture gamma ray energy spectrum resulting from irradiation of the borehole fluid and surrounding earth formations with high energy neutrons. Such external spectra, however, are often so variable that the hydrogen or other elemental spectral lines cannot be reliably located.
The prior art has also attempted to provide a reference line by use of a check source, i.e., a small sample of radioactive material with a fixed known gamma ray energy emission, which is placed close to or attached to the scintillator detector. For example, the use of a .sup.65 Zn check source for such purpose is disclosed in U.S. Pat. No. 4,031,367. However, since only certain gamma ray energies are conveniently so obtained, and since these gamma rays sometimes interfere with the spectrum to be measured, a check source is not always available or desirable.